1. Field of the Invention
The present invention relates to configurations of digital transmission apparatuses and configurations of communication networks using the digital transmission apparatuses, and more particularly to configurations of transmission apparatuses for use in a synchronous digital hierarchy and configurations of communication networks using the multiplex transmission apparatuses.
2. Description of the Related Art
In today""s digital transmission networks, the technology of synchronization has been advanced to such a degree that a communication network is synchronized with faster transmission apparatuses employing optical transmission. For functions and configurations of the digital transmission networks and transmission apparatuses, worldwide standards have been established such that a transmission apparatus and/or a communication network may be introduced in conformity to the standards to provide high quality transmission anywhere in the world. Examples of specific standards may include the standard (established in 1988) on a transmission system referred to as xe2x80x9cSDHxe2x80x9d (Synchronous Digital Hierarchy) defined in Recommendation G. 707 and so on by International Telecommunication Union (hereinafter abbreviated as xe2x80x9cITU-T), and the standard (established in 1991) on a transmission system referred to as xe2x80x9cSONETxe2x80x9d (Synchronous Optical Network) defined in Standard T1.105 by American National Standard Institute (hereinafter abbreviated as xe2x80x9cANSIxe2x80x9d), both of which define the configuration of optical synchronous communication systems and functions of transmission apparatuses.
Both SDH and SONET define the processing (transmission, multiplexing/demultiplexing, and so on) of a synchronous multiplexed signal (frame) which comprises a main signal portion referred to as a xe2x80x9cpayloadxe2x80x9d in which digitized main signals are multiplexed, and signals referred to as xe2x80x9coverheadsxe2x80x9d, added to the payload, for administration and maintenance operations for a transmission apparatus and communication network. The overheads include pointers which are used to perform stuff controls such as frame phase synchronization and frequency adjustment to provide a transmission system which has a less transmission delay and a higher administration and maintenance operation performance than conventional digital synchronous transmission apparatuses. The overheads added to the frame are classified into a section overhead (SOH) and a line overhead (LOH). The section overhead is used for administration and maintenance operations for each transmission span between transmission apparatuses and regenerators (defined as a section), and generated in an apparatus (including a regenerator), transmitted through a transmission span, and terminated at a next apparatus. The line overhead is used for administration and maintenance operations for each transmission interval between transmission apparatuses which process multiplexed main signals (defined as a line). The line overhead is generated in a transmission apparatus, transmitted through transmission spans and regenerators, and terminated at a next multiplexing apparatus. Examples of transmission apparatuses and networks adopting the above-mentioned SDH or SONET are described in JP-A-4-79628 and JP-A-5-114892.
In a transmission network in conformity to SDH or SONET, multiplexers may be occasionally connected to each other on a transmission line through regenerators. For carrying out the administration and maintenance operations between the multiplexers in such a transmission network, the line overhead is used to transmit and receive data and speech signals necessary to the administration and maintenance operations between the multiplexers. For example, D-bytes referred to as xe2x80x9cdata communication channelsxe2x80x9d and E-bytes referred to as xe2x80x9corderwiresxe2x80x9d of the line overhead are used to transmit and receive such data and speech signals between the multiplexers. Specifically, when a multiplexer on the transmission side inserts data and speech information into D4-D12 bytes and E2 byte of a line overhead and transmits the line overhead onto a transmission line, the line overhead is terminated at a destination multiplexer through the transmission line and regenerators, thereby carrying out the administration and maintenance operations between the multiplexers.
As the number of subscribers increases in the transmission network or as an increased amount of signals is communicated through the transmission network, extension and/or modifications in the transmission network, such as installation of additional multiplexers and replacement to the transmission network to a faster transmission line, may be required for supporting the increase in subscribers and the amount of communicated signals. For example, if the amount of communications increases, the existing transmission network is reconfigured, wherein similar multiplexers to those so far used are additionally installed in the transmission network, and a plurality of faster and larger-scaled multiplexers are introduced for processing signals in place of regenerators to modify the transmission network so that the multiplexers are connected through faster transmission line in the transmission network.
However, the reconfiguration of the transmission networks as mentioned above results in a problem involved in SDH and SONET standards. Specifically, since the line overhead is terminated at each transmitter according to SDH and SONET standards, an overhead outputted from an existing multiplexer may be terminated at an additionally installed faster multiplexer, so that the overhead is not transmitted to a far-end multiplexer which has so far received this overhead. In other words, the reconfiguration results in a lack of the administration and maintenance operations previously performed between the multiplexers before the reconfiguration. Thus, while the transmission capability of the transmission network for transmitting main signals is improved by the reconfiguration of the transmission network, the reconfiguration causes changes in the administration and maintenance operation capability of the transmission network, such as lack of the administration and maintenance operations so far performed between transmission apparatuses. Since administration and maintenance operation information previously provided is no longer available to a craft person dedicated to the maintenance of the transmission network, the craft person may suffer from quite inconvenient situations.
It is an object of the present invention to prevent changes in administration and maintenance operation capability due to a reconfiguration of a transmission network as mentioned above, and specifically to provide a transmission apparatus and a transmission network having an administration and maintenance operation capability which is not affected by any modification to the transmission network or which enable more flexible and high performance administration and maintenance operations in a simple configuration.
More specifically, the present invention provides a transmission apparatus which additionally has a function of passing through an overhead instead of processing the overhead standardized by SDH and SONET. The present invention also provides a transmission network, which is flexible and superior in administration and maintenance operation performance, wherein the transmission network uses transmission apparatuses as mentioned above, such that arbitrary transmission apparatuses in the transmission network are permitted to transmit and receive an arbitrary overhead therebetween. The present invention also provides a method of operating the configuration for the transmission network.
In a more detailed aspect, the present invention provides circuits and apparatus for use in a digital transmission apparatus, in a simple structure, for selectively cross-connecting a received tributary overhead and transmitting the cross-connected overhead to a far-end transmission apparatus. The present invention also allows for a tributary overhead containing information which cannot be interpreted at a far-end destination apparatus simply by passing the overhead through intervening apparatuses, and provides circuits and apparatus, in a simple structure, for converting a tributary overhead into information usable by a far-end destination apparatus and for transmitting the converted information to the far-end destination apparatus. In particular, the present invention provides simple circuits and apparatus for accurately notifying the number of transmission errors, if any, in a tributary through which an overhead is transmitted and received, to enable the management of transmission quality.
Moreover, the present invention provides a method of selectively cross-connecting an overhead and transmitting and receiving the cross-connected overhead between selected multiplexers, and a method of detecting and notifying transmission errors which have occurred in a tributary.
To solve the problems mentioned above, a multiplex transmission apparatus according to the present invention receives a multiplexed tributary signal comprising a payload having a plurality of main signals multiplexed therein and overhead bytes including a plurality of maintenance information associated with administration and maintenance operations, performs termination processing for the administration and maintenance operation information and transmission processing for the payload, thereafter converts the multiplexed tributary signal into a multiplexed high-speed signal comprising a payload including main signals which have been processed for transmission and a plurality of administration and maintenance operation information, and transmits the multiplexed high-speed signal. The multiplex transmission apparatus comprises a circuit or a apparatus for selecting predetermined maintenance information from the plurality of maintenance information included in the received multiplexed tributary signal, and inserting the predetermined maintenance information into an overhead byte in the multiplexed high-speed signal on the high-speed transmission side to thereby pass the predetermined maintenance information through the multiplex transmission apparatus.
Specifically, the multiplex transmission apparatus comprises an overhead passing circuit or a passing apparatus for passing maintenance information which is composed of an extraction circuit or an extraction apparatus for extracting predetermined maintenance information from the plurality of maintenance information included in the received multiplexed tributary signal, and an insertion circuit or an insertion apparatus for inserting extracted maintenance information into a predetermined location in the overhead bytes of the multiplexed high-speed signal to be transmitted. The overhead passing circuit or apparatus may be additionally provided with an cross-connecting circuit or cross-connecting apparatus for cross-connecting extracted maintenance information.
Also, the multiplex transmission apparatus receives a plurality of multiplexed tributary signals each comprising a payload and overhead bytes including a plurality of maintenance information associated with administration and maintenance operations, performs termination processing for the plurality of maintenance information and multiplexing of the plurality of payloads in a payload having a larger multiplexing degree, converts the plurality of multiplexed tributary signals into a multiplexed high-speed signal comprising the larger payload and overhead bytes, added to the large payload, having a size larger than the overhead bytes on the tributary side including a plurality of maintenance information associated with administration and maintenance operations, and transmits the multiplexed high-speed signal. The multiplex transmission apparatus comprises an overhead passing circuit or a passing apparatus for passing maintenance information which is composed of an extraction circuit or an extraction apparatus for extracting predetermined maintenance information from the plurality of tributary maintenance information, an cross-connecting circuit or a cross-connecting apparatus for cross-connecting information extracted by the extraction circuit or apparatus, and an insertion circuit or an insertion apparatus for inserting an output of the cross-connecting circuit or apparatus into a predetermined location of the high-speed overhead bytes, wherein the maintenance information received from the plurality of transmission paths is collectively transferred or passed.
A far-end transmission apparatus connecting this multiplex transmission apparatus through high-speed transmission line comprises an overhead passing circuit or passing apparatus which is composed of an extraction circuit or an extraction apparatus for extracting predetermined maintenance information from a plurality of high-speed maintenance information, a cross-connecting circuit or a cross-connecting apparatus for cross-connecting information extracted by the extraction circuit or apparatus, and an insertion circuit or an insertion apparatus for inserting an output of the cross-connecting circuit or apparatus into a predetermined location of the tributary overhead bytes, wherein upon receiving high-speed overhead bytes including maintenance information collectively transferred thereto, these signals are extracted, cross-connected, and inserted into predetermined locations of a plurality of tributary overhead bytes to be transmitted onto a plurality of tributary transmission lines.
Here, each of the foregoing multiplex transmission apparatuses processes the multiplexed signals defined in Recommendation G. 707 of International Telecommunication Union or in Standard T1. 105 of American National Standard Institute. Maintenance information to be passed is that included in a section overhead and a line overhead of tributaries. The maintenance information to be passed is transferred in a line overhead of a multiplexed high-speed signal. In addition, when these multiplexers are directly connected without regenerators, a section overhead is also used to pass or transfer an increased amount of maintenance information. When E byte, F byte, D byte, K byte, and Z byte, defined by the standard, are to be passed as maintenance information, these bytes are selected and passed as they are.
On the other hand, as a configuration for passing information on number of errors on a transmission path such as B bytes defined by the standard, a transfer circuit or a transfer apparatus is provided as follows. The transfer circuit or apparatus, upon detecting the number of errors which have occurred on a receiving tributary transmission line, inserts this number of occurring errors into an overhead byte of a multiplexed high-speed signal such that the number of errors is transferred in the multiplexed high-speed signal. Then, a far-end multiplex transmission apparatus which receives transferred maintenance information comprises an extraction circuit or an extraction apparatus for extracting the number of transmission errors of near-end tributary, an adder circuit or an adding apparatus for adding the extracted number of transmission errors of the near-end tributary and the number of high-speed transmission errors detected by this multiplex transmission apparatus, and a circuit or an apparatus for inserting the addition result into a second high-speed signal overhead and transferring the overhead, or for adding the number of transmission errors corresponding to the addition result to an error detecting signal of tributary. For another configuration or method, a multiplex transmission apparatus located in the middle of a high-speed transmission line transfer the number of transmission errors in tributary, and a multiplex transmission apparatus comprises detector circuit or a detecting apparatus for detecting the number of errors which have occurred in a high-speed transmission lines, an extraction circuit or an extraction apparatus for extracting the number of transmission errors of the near-end tributary, an adder circuit or an adding apparatus for adding the extracted number of transmission errors and the number of transmission errors detected by the detector circuit or apparatus, and a circuit or an apparatus for adding the number of transmission errors corresponding to the addition result to an error detecting signal of tributary.
Furthermore, to solve the problems mentioned above, a multiplex transmission network according to the present invention employs the apparatus as described above for a multiplex transmission apparatus for passing arbitrary maintenance information of the tributaries through intermediate multiplexers between arbitrary multiplex transmission apparatuses, so that the arbitrary multiplex transmission apparatuses transmit and receive arbitrary maintenance information of the tributaries therebetween.